This invention relates to start-up power supplies for television receivers.
Rectified AC line voltage is filtered and regulated to provide a high B+ operating voltage to the television receiver. The high B+ may be coupled, for example, to the horizontal deflection circuit for generating scanning current in the horizontal deflection winding. Low B+ voltages must also be provided as operating voltages to various receiver circuits, such as the oscillator and driver stages of the horizontal deflection circuit itself.
Often it is desirable to derive the low B+ voltages from a secondary winding of the horizontal output transformer after the horizontal deflection circuit has begun to function. However, during the initial start-up interval after the receiver is turned on, no low B+ voltages are generated by the horizontal output transformer to operate the oscillator and driver stages of the horizontal deflection circuit. Another source of low B+ voltage must be provided during this initial start-up interval.
A method of providing a low B+ start-up voltage is disclosed in U.S. patent application Ser. No. 750,632, filed Dec. 15, 1976, entitled, INRUSH CURRENT START-UP CIRCUIT FOR A TELEVISION RECEIVER, by R. E. Fernsler et al. AC line voltage is rectified and coupled to a smoothing filter for providing a high B+ voltage. When the receiver is turned on, an initial interval exists in which inrush current flows into the uncharged filter capacitor. A start-up circuit includes a winding coupled magnetically to an inductor in the path of the inrush current. The winding develops an alternating current potential during this initial interval, which is rectified to provide a low B+ start-up operating voltage for television receiver circuits, such as the horizontal deflection circuits.
After the initial interval has elapsed during the steady state interval, the horizontal deflection circuit itself will provide the low B+ operating voltage. The rectifiers in the start-up circuit should become reverse-biased by the steady state positive low B+ operating voltage, decoupling the alternating current potential appearing across the aforementioned winding from the television receiver circuits during the steady state interval.
For certain television receivers, a relatively large start-up power is needed by the horizontal deflection circuits. Under these circumstances, the alternating current potential may be of sufficient magnitude to forward-bias the start-up circuit rectifiers during peak excursions of the alternating current potential even during the steady state interval. This potential, when coupled to the horizontal deflection circuit, will produce unwanted modulation of the raster. It is desirable, therefore, when designing a start-up circuit which is required to deliver relatively large start-up power, to ensure that the alternating current potential be decoupled from the television receiver circuits for the entirety of the steady state interval.